Distributed inductively-coupled plasma source and circuit for coupling induction coils to RF power supply

ABSTRACT

Apparatus and method for inductively coupling electrical power to a plasma in a semiconductor process chamber. In a first aspect, an array of wedge-shaped induction coils are distributed around a circle. The sides of adjacent coils are parallel, thereby enhancing the radial uniformity of the magnetic field produced by the array. In a second aspect, electrostatic coupling between the induction coils and the plasma is minimized by connecting each induction coil to the power supply so that the turn of wire of the coil which is nearest to the plasma is near electrical ground potential. In one embodiment, the hot end of one coil is connected to the unbalanced output of an RF power supply, and the hot end of the other coil is connected to electrical ground through a capacitor which resonates with the latter coil at the frequency of the RF power supply.

CROSS REFERENCE TO RELATED APPLICATION

This patent application is a divisional of application Ser. No.09/039,216 filed Mar. 14, 1998, now U.S. Pat. No. 6,273,022.

FIELD OF THE INVENTION

This invention relates to inductively coupled plasma sources forsemiconductor process chambers.

BACKGROUND OF THE INVENTION

Many processes for fabricating semiconductors, such as etching anddeposition, are plasma-enhanced or plasma-assisted; that is, they employprocess reagents excited to a plasma state within a vacuum chamber.Generally, the plasma is excited by coupling radio frequency (RF)electrical power to the process gas mixture. The RF electrical fielddissociates atoms in the process gas mixture to form the plasma.

One method of coupling RF power to the process gases is inductivecoupling, in which an RF power supply is connected to an induction coilwhich is mounted either inside the chamber or just outside a portion ofthe chamber wall which is dielectric. In comparison with a capacitivelycoupled plasma source, an advantage of an inductively coupled plasmasource is that it permits adjusting the RF power supplied to the plasmaindependently of the DC bias voltage on the semiconductor workpiece.

Induction coils commonly are shaped as a solenoid which either encirclesthe cylindrical side wall of the vacuum chamber, or else is mounted onthe circular top wall of the chamber. Other conventional induction coilsare shaped as a planar or semi-planar spiral mounted on the circularflat or dome-shaped top wall of the chamber. The solenoid and spiralcoils share the disadvantage of producing an RF electromagnetic fieldwhich extends along the axis of the coil toward the semiconductorworkpiece. A large RF field near the workpiece can be undesirablebecause it possibly can damage the semiconductor devices beingfabricated on the workpiece.

U.S. Pat. No. 5,435,881 issued Jul. 25, 1995 to Ogle discloses aninductively coupled plasma source which minimizes the RF magnetic fieldnear the semiconductor workpiece. It employs an array of induction coilsdistributed over the dielectric, circular top wall of a process chamber.The axis of each coil is perpendicular to the chamber top wall and tothe semiconductor workpiece, and adjacent coils are connected out ofphase so as to produce opposite polarity magnetic fields. Thisarrangement produces a “cusp” magnetic field pattern in the “near field”adjacent the top wall, which excites the process gases to a plasmastate. However, in the “far field” near the workpiece, the oppositepolarity magnetic fields cancel out so that the magnetic field strengthnear the workpiece is negligible, thereby minimizing any risk of damageto the semiconductor devices being fabricated.

One disadvantage of the Ogle design is that the RF magnetic field isnon-uniform near the perimeter of the induction coil array.Specifically, the perimeter of Ogle's magnet array deviates from thecentral pattern of evenly spaced, alternating polarity, magnetic poles.Such spatial non-uniformity in the RF field can produce undesirablespatial non-uniformities in the plasma-enhanced semiconductorfabrication process.

SUMMARY OF THE INVENTION

The present invention is an apparatus and method for inductivelycoupling electrical power to a plasma in a semiconductor processchamber.

In a first aspect, the invention comprises an array of induction coilsdistributed over a geometric surface having a circular transversesection. Uniquely, each coil has a transverse section which iswedge-shaped so that the adjacent sides of any two adjacent coils in thearray are approximately parallel to a radius of the circular transversesection of the geometric surface.

The invention can produce a plasma adjacent a semiconductor workpiece ina plasma chamber having excellent spatial uniformity, i.e., uniformityin both the radial dimension and the azimuthal dimension. The plasma hasexcellent radial uniformity because the adjacent sides of adjacent coilsare approximately parallel. It has excellent azimuthal uniformitybecause the coils are equally spaced azimuthally relative to thegeometric surface.

Our invention can be adapted to operate over a wide range of chamberpressures. Some conventional designs couple energy to the plasma bycontinuously accelerating electrons at a resonant frequency, which canbe achieved only at chamber pressures low enough to ensure that the meanfree path of the electrons is greater than the spacing between themagnetic poles. In contrast, our invention does not require continuousacceleration of electrons, so it is not restricted to operation at lowchamber pressures.

Our invention readily can be adapted to larger or differently shapedplasma chambers by adding induction coils to the array. It isstraightforward to optimize our design for different processes anddifferent chamber sizes and shapes, because the plasma enhancementcontributed by any two adjacent coils is localized to the vicinity ofthe two coils. In contrast, it typically is much less straightforward toscale conventional designs which employ a single induction coil.

Preferably, the geometric surface is the surface of a flat, circulardielectric wall at one end of the process chamber, and the array ofcoils is mounted on the exterior surface of this wall. Alternatively,the array of coils can be mounted inside the vacuum chamber, in whichcase the geometric surface typically would not be a physical object, butmerely a geometric shape.

In the preferred embodiments, adjacent coils produce magnetic fields ofopposite polarity. Advantageously, in contrast with many conventionalinduction coil designs, the eddy currents induced by adjacent coils willtend to cancel out each other rather than additively reinforcing eachother, so that no eddy current will circulate around the perimeter ofthe chamber wall 12.

The foregoing embodiment of the invention is ideal for cylindricalplasma chambers for processing circular semiconductor wafers. Inalternative embodiment ideally suited for processing rectangularworkpieces such as flat panel displays, the induction coils are arrangedin a rectangular array or matrix rather than in a circular array. In arectangular array, the coils need not be of a specific shape, and can becircular or rectangular in transverse section, for example. To maximizethe lateral uniformity of the plasma, the lateral or transverse spacing“W” between the perimeters of adjacent coils should be equal for everypair of adjacent coils. The coils are connected to an RF power supplywith respective polarities such that adjacent coils produce RF magneticfields of opposite polarity.

In a second aspect of the invention, each induction coil is connected tothe power supply in such a way that the turn of wire of the coil whichis closest to the plasma is at or near electrical ground potential. Thisaspect of the invention minimizes capacitive (electrostatic) couplingbetween the induction coils and the plasma, thereby minimizingsputtering of the chamber wall adjacent the coils.

In one embodiment, the end of each coil which is closest to the plasmais connected directly to electrical ground, and the opposite end of thecoil is connected to an unbalanced output of an RF power supply. In asecond and a third embodiment, two coils are connected in series byconnecting together the end of each coil which is closest to the plasma.In the second embodiment, the opposite (“RF hot”) end of each coil isconnected to a respective balanced output of an RF power supply. In thethird embodiment, the hot end of one coil is connected to the unbalancedoutput of an RF power supply, and the hot end of the other coil isconnected to electrical ground through a capacitor which resonates withthe latter coil at the frequency of the RF power supply.

A third aspect of the invention is the circuit used in the thirdembodiment of the preceding paragraph for coupling two coils to anunbalanced power supply output so as to maintain the junction betweenthe two coils close to electrical ground potential. This circuit isnovel and valuable independently of whether the coils are associatedwith a plasma chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially schematic, sectional, side view of a plasmachamber employing the invention.

FIG. 2 is a schematic top view of the array of induction coils in thechamber.

FIG. 3 is a partially schematic, perspective view of the induction coilarray of FIG. 2.

FIG. 4 is a perspective view of one of the induction coils of FIGS. 1-3.

FIG. 5 is a sectional view of one of the induction coils of FIGS. 1-3.

FIG. 6 is an exploded perspective view of the induction coil of FIG. 5.

FIG. 7 is a sectional view of the induction coil array along asemi-circular section path showing the azimuthally oriented magneticcusp field.

FIG. 8 is an electrical schematic diagram of two adjacent coilsconnected in parallel between an RF power supply and electrical ground.

FIG. 9 is an electrical schematic diagrams of two series-connectedadjacent coils connected to a balanced output of an RF power supply.

FIG. 10 is an electrical schematic diagram of two series-connectedadjacent coils connected to an unbalanced output of an RF power supply,including a novel feature in which one coil is connected to electricalground through a capacitor.

FIG. 11 is a schematic top view of an alternative induction coil arrayincluding a center coil.

FIG. 12 is a partially schematic, perspective view of the induction coilarray of FIG. 11.

FIG. 13 is a schematic top view of an alternative induction coil arrayhaving concentric coils.

FIG. 14 is a schematic top view of a rectangular array of cylindricalinduction coils.

FIG. 15 is a schematic top view of a rectangular array of inductioncoils, wherein each coil has a rectangular transverse cross section.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a plasma chamber utilizing the present invention. Theillustrated chamber is intended for plasma-enhanced etching ofdielectric films on silicon wafers, but the invention is equally usefulin any plasma chamber used for semiconductor fabrication processes suchas etching, chemical vapor deposition, or sputter deposition.

The vacuum chamber has a cylindrical aluminum side wall 12, a circularaluminum bottom wall 14, and a circular top wall or lid 10 composed of adielectric material such as aluminum oxide (Al₂O₃, commonly calledalumina), aluminum nitride, or silicon carbide. We prefer alumina,primarily because it has been used successfully as a dielectric in manyother plasma chamber designs, and also because it is much less expensivethan the other contemplated dielectric materials.

The chamber side wall and bottom wall are electrically grounded. Analuminum cathode electrode 16 is oriented perpendicular to thecylindrical axis of the chamber and is electrically insulated from thegrounded chamber walls by dielectric support shelf 18. A semiconductorworkpiece such as a silicon wafer (not shown) is mounted on the top faceof the cathode electrode by conventional means such as a mechanicalclamp ring or an electrostatic chuck. Such chamber construction iscompletely conventional.

Process gases 22 flow into the chamber through several gas inlet ports(not shown) which are azimuthally spaced around the chamber side wall12, below the lid 10. First and second RF power supplies 32 and 34couple RF power to the chamber interior 100 so as to form a plasma fromthe process gases.

An exhaust pump, not shown, is mounted outside the exhaust port 24 inthe chamber bottom wall 14. The pump exhausts the process gases andreaction products from the chamber through sinuous exhaust baffle 25,and then out the exhaust port 24.

The sinuous exhaust path imposed by the preferred exhaust baffle 25functions to block the plasma from reaching the exhaust port. Theexhaust baffle consists of overlapping lateral extensions of anodizedaluminum outer liner 26 and inner liner 27. The liners are removable tofacilitate cleaning. The sinuous exhaust baffle and liners are describedmore fully in commonly assigned U.S. Pat. No. 5,891,350 issued Apr. 6,1999 to Shan et al., the entire content of which is hereby incorporatedby reference into this patent application.

The upper end of the outer liner 26 includes an inwardly extending,annular shelf 28 on which the lid 10 rests, with an intervening O-ring29 to provide a vacuum seal. A gas passage in the outer liner providesflow of process gas 22 to the inlet ports.

The first RF power supply 32 supplies electrical power through a firstimpedance matching network 31 to a novel array 30 of induction coilsmounted on the dielectric chamber lid 10. Each coil 40, 42 in the array30 is wound around an axis which is parallel to the chamber axis, i.e.,perpendicular to the chamber lid and to the semiconductor workpiece. TheRF current through the induction coils produces an RF electromagneticfield in the region of chamber just below the lid so as to couple RFpower to the plasma and thereby enhance the density of the plasma. (FIG.7 depicts the RF magnetic field lines 120.)

Conventionally, a second impedance matching network 33 capacitivelycouples the second RF power supply 34 to the cathode electrode 16 so asto produce a negative DC bias voltage on the cathode electrode relativeto the electrically grounded chamber walls. The negative bias voltage onthe cathode electrode 16 attracts ions from the process gas mixturetoward the semiconductor workpiece so that the process gases perform adesired semiconductor fabrication process on the surface of theworkpiece, such as a conventional process for etching an existing filmon the workpiece surface or for depositing a new film on the surface.

FIGS. 2-4 and 6 depict the wedge shape of each of the eight inductioncoils 40, 42 in the array 30, the coils being equally spaced around theazimuth of the chamber lid 10. As shown in FIGS. 4-7, each coil 40, 42has a number of turns of copper wire 43 wound around a hollow coil form50. Each coil form has a wedge-shaped top surface 54 as shown in FIGS.2-4, and each has a U-shaped cross section when viewed from the side asshown in FIGS. 4-6. Specifically, each coil form 50 consists of acurved, rectangular, wide outer surface 53; an almost triangular,wedge-shaped top surface 54; and a curved, narrow, inner tip surface 55.The azimuthal sides 44 of each coil form are completely open, as shownin FIGS. 4-7.

Each coil form 50 is composed of dielectric material so that the coilwindings can touch the form without being electrically shorted. The coilforms preferably should have a very low dielectric constant so as tominimize the parasitic capacitance across the coil windings, and therebyraise the self-resonant frequency of the coils. Our preferred materialfor the coil forms is Teflon.

To facilitate mounting or removing the induction coil array as a singleunit without opening the chamber lid 10, the eight wedge-shaped coilforms 50 are attached by push-on fasteners 51 to a single, disc-shapedbase 56. In the preferred embodiment, the base 56 is composed of aplastic material having higher mechanical rigidity than the Teflon usedfor the coil forms. The preferred plastic is sold by Dupont under thetrademark Ultem. It commonly is called “natural Ultem” to distinguish itfrom “black Ultem” which includes carbon.

Preferably, each induction coil 40, 42 encircles a magnetic core 52. Webelieve the magnetic cores 52 concentrate and shape the magnetic fieldso that the magnetic flux lines 120 extending between adjacent coils 40,42 will be concentrated primarily in an arc extending through theinterior 100 of the plasma chamber between the respective lower axialends of each coil as shown in FIG. 7. Without the magnetic cores 52, agreater proportion of the magnetic flux would extend laterally betweenthe central turns 43 of the coils above the lid 10. In other words, webelieve the magnetic cores 52 concentrate the magnetic flux 120 in theregion of the chamber interior 100 occupied by the plasma, therebyimproving the efficiency with which the induction coils 40, 42 couple RFpower to the plasma.

In the preferred embodiment, the magnetic core of each induction coilconsists of twelve bars 52 of magnesium zinc soft ferrite having amagnetic permeability of approximately 40 to 60. (The vendor of theferrite we tested claims the permeability is 60, but reference booksgenerally show a permeability of 40 for magnesium zinc ferrite.)

To permit air cooling of the ferrite bars, the ferrite bars are spacedapart. The gaps between ferrite bars are occupied by air. To maintainthe spacing and alignment of the ferrite bars, the top of each baroccupies a distinct opening in the top 54 of the coil form 50, and thebottom of each bar occupies a distinct opening in the base plate 56. Theopenings in the base plate also have the advantage of lowering thebottom of each ferrite bar so as to be closer to the chamber lid 10,thereby maximizing the strength of the magnetic field produced in thechamber by the coil array 30.

The coils are connected to the first RF power supply 32 in such a waythat, during any half-cycle of the RF current waveform, the current flowthrough half the coils 40 is clockwise, and the current flow through theother half of the coils 42 is counterclockwise, with the coils of thetwo polarities being positioned alternately around the azimuth of thearray. FIGS. 2 and 7 illustrate this alternating arrangement bylabelling successive coils 40, 42 alternately as S and N, respectively,to represent the magnetic field produced by each respective coil duringone half-cycle of the RF current waveform. During the next half-cycle,the south and north magnetic poles will be interchanged.

The alternating polarity magnetic fields from the array 30 of coilsproduces in the “near field” (i.e., near the chamber lid 10) a resultantmagnetic field having a “cusp” pattern, as depicted by magnetic fieldlines 120 in FIG. 7. Consequently, the electrical power provided by theRF power supply 32 is coupled to the process gas mixture adjacent thechamber lid 10 so as to enhance the density of the plasma.

Each coil is wedge-shaped so that the adjacent sides 44 of any twoadjacent coils are approximately parallel. Except at the center of thelid, the azimuthal gap “W” between two azimuthally adjacent coils isapproximately the same at all radial positions, so that the coilsproduce a magnetic field with excellent radial uniformity.

At increasing distances from the bottom of the induction coil array 30,the magnetic field strength rapidly drops to negligible levels, becausethe magnetic field lines of opposite polarity cancel each other out. Werefer to as the “far field” the region within the chamber which issufficiently far from the bottom of the coil array that the magneticfield strength is at least one or two orders of magnitude less than themagnetic field strength adjacent the lid. We refer to as the“penetration distance” the distance the magnetic cusp field 120 extendsinto the chamber; i.e., the penetration distance is the depth of the“near field”.

Preferably, the semiconductor workpiece (mounted on the cathodeelectrode 16) is far enough from the induction coil array 30 to belocated in the far field where the magnetic field strength isnegligible. This is advantageous for two reasons.

First, minimizing the magnetic field at the semiconductor workpiecesometimes can help minimize the risk of damaging the semiconductordevices being fabricated on the workpiece, although the importance ofminimizing the magnetic field also depends on other parameters of thesemiconductor fabrication process being performed and on the dielectricand semiconductor structures already fabricated on the workpiece.

Second, locating the workpiece a distance from the coil array which issubstantially beyond the penetration distance smooths out the effects oflocalized spatial nonuniformities in the magnetic field strength in thenear field. Specifically, we observe the plasma glow is brightest,indicating the magnetic field is strongest, near the gaps betweenadjacent coils. If the workpiece were positioned too close to the coilarray, the process rate on the workpiece would exhibit peaks and valleyscorresponding to the locations of the gaps between coils and the coilcenters, respectively. Conversely, at increasing distances substantiallybeyond the penetration distance of the magnetic field, the plasmadensity becomes progressively more uniform due to diffusion. Therefore,when the workpiece is positioned substantially beyond the penetrationdistance, the semiconductor fabrication process can achieve excellentspatial uniformity.

Because the induction coil array enhances the plasma density in the nearfield, the plasma density in the far field (substantially beyond thepenetration distance of the magnetic field from the coil array)generally is lower than the plasma density in the near field.Consequently, the process rate on the workpiece will be undesirablyreduced if the workpiece is positioned greater distances beyond thepenetration distance. For example, in tests performed using a processfor etching a silicon oxide film on a silicon wafer, we found that theetch rate desirably increased as we positioned the coil array and theworkpiece (wafer) closer together. Therefore, designing the penetrationdistance of the coil array and the mounting location of the workpieceinvolves a balancing between improving the process rate when theworkpiece is closer to the penetration distance and improving theprocess spatial uniformity when the workpiece is farther beyond thepenetration distance.

The plasma density has a radial distribution in the vicinity of the coilarray 30 which is quite different from its radial distribution in thevicinity of the workpiece. Specifically, as ions from the plasma migratefrom the vicinity of the coil array toward the workpiece, the plasmadensity near the perimeter of the chamber (and the perimeter of theworkpiece) tends to be reduced by recombination of ions at the chamberside wall 12. Consequently, in order to maximize the radial uniformityof the plasma density in the vicinity of the workpiece, the magneticfield pattern of the coil array preferably should produce a plasma whosedensity is radially non-uniform: specifically, whose density is strongernear the perimeter of the coil array than at the center of the coilarray. Accordingly, in the illustrated preferred embodiment the coilarray 30 has a central region with no induction coils so as to produce amaximum RF magnetic field near the perimeter of the coil array. Lateraldiffusion as the plasma species migrate from the vicinity of the coilarray toward the workpiece augments the plasma density near the centeraxis and results in a radially uniform plasma density adjacent theworkpiece.

To further maximize the plasma density near the chamber side wall 12,the diameter of the coil array preferably is close to or greater thanthe diameter of the chamber. In the illustrated preferred embodiment,this is accomplished by positioning the outer edge 53 of the coils asclose as possible to the cover 58. Advantageously, our design permitspositioning the coils close to the cover 58 or chamber side wall 12without inducing large eddy currents in the cover or side wall.

Eddy currents are undesirable because they dissipate power from the RFpower supply 32 as heat, thereby reducing the power coupled to theplasma. In our design, to the extent an individual coil 40 or 42 tendsto induce eddy current in the chamber wall, the cumulative eddy currentinduced by the entire coil array 30 is minimized because adjacent coilshave magnetic fields of opposite polarity, and therefore induce eddycurrents of opposite polarity. Therefore, in contrast with manyconventional induction coil designs, the eddy currents induced byadjacent coils will tend to cancel out each other rather than additivelyreinforcing each other, so that no eddy current will circulate aroundthe perimeter of the chamber wall 12.

The illustrated preferred embodiment was implemented for use in a plasmachamber for fabricating 8-inch diameter silicon wafers. In thisembodiment, the diameter of the coil array is 12 inches (30 cm), and theaxial length or height of each coil is 2 inches (5 cm). The gap Wbetween parallel faces 44 of adjacent coils is 1.25 inch (3.2 cm). Thediameter D of the central region of the coil array, between tips 55 ofopposite coils, is 3.9 inches (9.9 cm). Each coil has 3¾ turns of copperwire 43. The alumina ceramic lid 10 is 12 inches (30 cm) in diameter and0.65 inch (1.65 cm) thick. The cathode electrode 16 supports an 8-inchsilicon wafer 3 inches (7.6 cm) below the lid.

In tests of this embodiment using a standard process for etching asilicon oxide layer on an 8-inch silicon wafer, the etching appearedspatially uniform, with no visually observable pattern etched on thewafer corresponding to the shape of the ferrite cores or thewedge-shaped coils. We measured a one-sigma spatial nonuniformity ofetch rate equal to only one percent (with 3 mm edge exclusion). Suchexcellent observed and measured etch rate uniformity implies that themagnetic field strength at the wafer was negligible, and that themagnetic field uniformity adjacent the lid was at least as good asnecessary to achieve excellent process uniformity on the wafer.

The shape and uniformity of the magnetic field are affected by therelative values of the following dimensions: the axial length or height“H” of the coils; the azimuthal gap “W” between adjacent coils; and thediameter “D” of the central area of the coil array which is not occupiedby a coil, i.e., the diameter bounded by the tips of radially oppositecoils (see FIGS. 2 and 5); and the angular center-to-center spacingbetween azimuthally adjacent coils.

The azimuthal gap W and the angular center-to-center spacing betweenazimuthally adjacent coils are important design parameters because theyaffect the “penetration distance” by which the magnetic cusp fieldextends below the chamber lid 10 into the chamber interior 100.Increasing the azimuthal gap W between coils or the angularcenter-to-center spacing between coils typically will increase thepenetration distance of the magnetic field. Optimizing the penetrationdistance of the magnetic field typically should be the primaryconsideration in selecting the azimuthal gaps when designing theinduction coil array 30.

The optimum penetration distance of the magnetic field balancesconsiderations of process rate and spatial uniformity. Increasing thepenetration distance can be advantageous because it increases the volumeof the plasma below the lid to which the RF power is coupled. However,decreasing the penetration distance can be advantageous by allowing thesemiconductor workpiece to be positioned closer to the coil array 30while still remaining in the far field where the magnetic field isnegligible. For a given penetration distance, the distance of theworkpiece beyond the penetration distance affects process rate andspatial uniformity as described above.

Even with the workpiece only 3 inches below the lid in the preferredembodiment just described, we found that the magnetic field strength atthe workpiece was negligible, as evidenced by the absence of significantazimuthal variation in etch rate in the tests just described. Therefore,the effective penetration distance of the magnetic field must have beenless than 3 inches.

The illustrated preferred embodiment employs eight wedge-shaped coils40, 42 spaced around the azimuth of the chamber lid 10. Accordingly, theangular center-to-center spacing between adjacent coils is 360°/8=45°.Increasing the number of coils, and thereby decreasing the angularspacing between coils, would be expected to produce two effects. First,it would improve the azimuthal uniformity of the plasma near theworkpiece. Second, if the azimuthal gap W between coils were reduced inproportion to the reduced angular spacing between coils, the penetrationdistance of the magnetic field into the chamber would be reduced asdescribed above.

The preceding discussion of adjusting the penetration distance of themagnetic field by adjusting the azimuthal gap W or the angularcenter-to-center spacing assumes that the magnetic field produced by thecoils appears primarily in the gaps between adjacent coils, as shown bythe field lines 120 in FIG. 7, rather than being directly under eachcoil as would be the case if adjacent coils were of the same polarityrather than opposite polarities.

In the preferred embodiment whose dimensions were stated above, theratio of the coil height H to azimuthal gap W is about 1.6, i.e.,H:W=1.6:1. Tests also were performed on an embodiment in which theazimuthal gap W was increased from 1.25 in. to 5 in., so that the gapwas 2.5 times greater than the coil height, thereby inverting the ratio,i.e., H:W=1:2.5. The tests showed areas of maximum etch rate on theareas of the workpiece directly under each coil. This indicates that themagnetic field produced by each coil was concentrated under the coilwhen H:W=1:2.5, rather than extending between adjacent coils as whenH:W=1.6:1.

Extrapolating from this result, we believe that H:W preferably should begreater than one—i.e., the axial coil length or height should be greaterthan the azimuthal gap W between adjacent coils—in order to produce amagnetic field pattern that extends uniformly across the gaps betweencoils rather than being concentrated directly below each coil. Webelieve such pattern will provide the best spatial uniformity of theplasma in the vicinity of the workpiece, and it will permit adjustingthe penetration distance of the magnetic field as described above.

If the diameter D of the central area is too large relative to theazimuthal gaps W between adjacent coils, there will be a drop inmagnetic field strength near the center of the coil array. As statedabove, we observed excellent process uniformity with D=3.9 in. andW=1.25 in. Conversely, if the diameter D is substantially reduced to avalue comparable to the azimuthal gap W, or even less, all the coilswill be close enough to the center to strongly interact in a complexmanner which we have not tested or analyzed. Therefore, we presentlycannot predict the pattern of the resulting magnetic field.

One advantage of our invention is that it is straightforward to optimizethe design for different processes and different chamber sizes andshapes, because the plasma enhancement contributed by any two adjacentcoils is localized to the vicinity of the two coils. For example, it iseasy to optimize our design to maximize spatial uniformity by increasingor decreasing the magnetic field produced by those coils closest to theareas of the semiconductor workpiece where the plasma density or processrate is lowest or highest. As another example, our design can be adaptedto a larger chamber by simply adding more coils and/or increasing thesize of each coil.

As yet another example, our design can be adapted to a rectangularchamber for manufacturing rectangular flat panel displays by arrangingthe coils in a rectangular array or matrix rather than in a circulararray. To maximize the lateral uniformity of the plasma density in thevicinity of the workpiece, the lateral or transverse spacing “W” betweenthe perimeters of adjacent coils should be equal for every pair ofadjacent coils. In the previously described chamber having a circulartransverse cross section, as would be used for fabricating circularsemiconductor wafers, such uniform spacing “W” is best achieved usingwedge-shaped coils. In a chamber having a rectangular transverse crosssection for fabricating a rectangular semiconductor workpiece, uniformspacing “W” can be achieved independently of the shape of each inductioncoil, provided the gaps “W” between the perimeters of the coils areuniform. FIG. 14 illustrates a rectangular array of induction coils 40,42 in which the coils are cylindrical, i.e., each coils has a circulartransverse section. FIG. 15 illustrates a rectangular array in whicheach coil has a rectangular transverse section.

Regulating the temperature of the lid 10 is important for at least tworeasons. First, the temperature of the lid (and of other chambersurfaces exposed to the plasma) strongly affects the performance of theplasma process. Therefore, the temperature of the lid should beregulated to ensure consistent process performance. Second, in processesthat unavoidably deposit polymers on the lid, excessive temperaturefluctuations can cause the polymer to flake off and contaminate theworkpiece.

The temperature of the lid will tend to rise while the plasma is presentin the chamber, due to absorption of heat from the plasma and absorptionof both heat and RF energy from the coils 40, 42. Conversely, thetemperature of the lid will fall while the plasma is off during thetimes that workpieces are unloaded and loaded into the chamber.

To regulate the temperature of the lid, our preferred embodiment of thelid includes channels (not shown in the drawings) through which we pumpa dielectric cooling fluid, preferably a mixture of deionized water andethylene glycol. An external control system regulates the temperature ofthe cooling fluid at 50° C.

If each coil 40, 42 includes a magnetic core 52 as in the illustratedpreferred embodiment, it also is important to regulate the temperatureof the magnetic cores, because the magnetic permeability of mostmagnetic materials is temperature dependent. For most ferrite materials,the magnetic permeability increases with temperature up to a maximumvalue at a certain threshold temperature, and then decreases attemperatures above the threshold. For the particular manganese-zincferrite used in our preferred embodiment, the threshold temperature isabout 100° C.

When the temperature of the magnetic core is above the thresholdtemperature, there is a risk of an uncontrolled increase in temperature(thermal runaway), because the increasing temperature of the magneticcores will decrease their magnetic permeability, which will decrease theinductance of each coil, which will increase the current through eachcoil, and hence will further increase the temperature of the cores. byincreasing the electrical current through the coil. To ensure againstsuch thermal runaway, it is highly preferable to maintain thetemperature of the magnetic cores below their threshold temperature.

Beyond maintaining the magnetic cores 52 below their thresholdtemperature, it is preferable to regulate the temperature of the coresto limit their temperature fluctuations as much as practical. Astemperature fluctuation changes the inductance of the coils 40, 42, thevariable inductors and/or variable capacitors in the impedance matchingnetwork 31 must be adjusted to maintain a constant level of RF powercoupled to the plasma from the RF power supply 32. Conventional matchingnetworks 31 can perform the necessary adjustments automatically andcontinuously. However, the wider the range over which the temperaturesof the magnetic cores are permitted to fluctuate, the wider theadjustment range required for the variable inductors and/or variablecapacitors in the impedance matching network, thereby increasing thecost of the variable inductors and/or variable capacitors. Therefore, tominimize the cost of the matching network, it is preferable to regulatethe temperature of the cores to limit their temperature fluctuations asmuch as practical.

In the presently preferred embodiment, we cool the ferrite cores 52 bytwo means. First, a fan (not shown) is mounted above the coil array 30and blows relatively cool, ambient air downward over the coil array. Thecooling is facilitated by the coil forms 50 being hollow and being openat their azimuthal sides 44, so that sides of the ferrite cores arecompletely exposed to the cool air. Second, the previously describedtemperature regulation of the chamber lid 10 helps cool the coil arraydue to heat transfer between the lid and both the coil forms 54 and thebase plate 56. Consequently, we maintain the temperature of the ferritecores within the range of 20° to 45° C.

It is conceivable that more aggressive cooling mechanisms may berequired in other applications requiring higher RF power. For suchapplications, we contemplate the possibility of employing forced gascooling of the coil array 30. Specifically, we contemplate pumping airor nitrogen gas into channels formed within the base plate 56, fromwhich the gas would flow upward over the coils 40, 42 and then beexhausted by a gas exhaust manifold positioned above the coil array.

Since the magnetic field strength declines rapidly with axial distanceaway from the coil array 30, the dielectric lid 10 which separates thecoil array from the plasma should be as thin as possible, although notso thin as to be easily cracked or otherwise damaged. As stated earlier,the lid in the presently preferred embodiment is 0.65 inch thick. Thereason the preferred lid is so thick is to accommodate the water coolingchannels just described. We expect that the lid thickness could bereduced to half this amount by omitting the cooling passages. However,we currently consider it impractical to omit the cooling channelsbecause of the importance of regulating the lid's temperature asdescribed above.

In the illustrated embodiment, the base 56 is not fastened directly tothe lid 10 because the alumina material of the lid is not strong enoughto reliably withstand the stress of being rigidly bolted to the coilarray. Instead, the base is attached to a metal cover 58, and the coveris attached to the chamber wall 12. The purpose of the cover is simplyto prevent RF radiation from the coils which may interfere with otherelectrical equipment nearby, and to protect people from the risk oftouching the coils and receiving an electrical shock. More specifically,the base is attached to the metal cover by four L-shaped, threaded,dielectric standoffs 57 which are fastened by screws 59 to correspondingmounting holes in the perimeter of the base 56 and in the cover 58.

To ensure consistency and repeatability of process performance, it isimportant to accurately align the center of the coil array 30 with thecentral axis of the chamber. Therefore, the standoffs, fasteners, andmounting holes just described, which determine the position of the base56 relative to the chamber, must have tight dimensional tolerances. Wefind it practical to maintain dimensional tolerances no greater than afew mils, i.e., about 0.1 mm.

The alignment of the lid 10 is less critical than the alignment of thecoil array, since the lid has no electrically active components. In theillustrated preferred embodiment, the perimeter of the lid simply restson the inwardly protruding shelf 28 of the outer liner 26. No fastenersare used; until the chamber is evacuated the lid is held in place onlyby its own weight. When a vacuum is created in the chamber, theatmospheric pressure on the outside of the lid holds the lid tightly inplace.

To maximize the magnetic field strength in the plasma chamber, it isdesirable to mount the base 56 as close as possible to the lid 10. Ourpreferred method of mounting the base allows it to rest directly on thelid. Specifically, the holes in the cover 58 through which screws 59extend are vertically elongated, thereby allowing the base 56 to movevertically relative to the cover before the screws are tightened. First,the entire coil array 30 is loosely attached to the cover by extendingscrews 59 through the holes in the cover and partially threading theminto the standoffs 57. Second, the resulting assembly is lowered ontothe chamber lid, so that the base 56 rests on the lid 10. Finally, thecover is bolted to the chamber, and the screws 59 are tightened torigidly maintain the alignment of the coil array.

FIGS. 8-10 respectively show three alternative circuits for producingopposite magnetic field polarities in each pair of adjacent coils 40,42. To simplify the drawings, FIGS. 8-9 show only one of the four coilpairs, and FIG. 10 shows only two of the four coil pairs. In all threecircuits of FIGS. 8-10, the four pairs of coils actually are connectedin parallel with each other to the output of the impedance matchingnetwork. FIG. 10 illustrates the parallel connection of the four coilpairs by showing two of the coil pairs 110 connected in parallel witheach other.

In the FIG. 8 design, all 8 coils 40, 42 are connected in parallel tothe power supply 32; hence the power supply must deliver to the coilarray a total current equal to 8 times the current through eachindividual coil. In the designs shown in FIGS. 9 and 10, each pair 110of adjacent coils 40 a, 42 is connected in series, and the resultingfour series-connected pairs of coils are connected in parallel to thepower supply 32. Consequently, the power supply must deliver to the coilarray a total current equal to four times the current through eachindividual coil. We consider the FIG. 8 design less desirable because itrequires the power supply to deliver twice the current required in thedesigns of FIGS. 9 and 10.

For many semiconductor fabrication processes, it is desirable for the RFpower from the first power supply 32 to be coupled to the plasmainductively rather than capacitively (i.e., electrostatically) to thegreatest extent possible, so as to minimize sputtering of the chamberlid by ions from the plasma. All of the wiring schemes shown in FIGS.8-10 share the advantage of maintaining the lowermost turn of each coilat, or close to, electrical ground potential. Consequently, the portionof each coil which is closest to the plasma will have the lowest RFvoltage, thereby reducing capacitive coupling between the coil and theplasma.

A conventional Faraday shield can be mounted between the induction coilarray and the chamber interior if it is desired to further reducecapacitive (electrostatic) coupling between the coils and the plasma.

In the FIG. 8 embodiment, the first coil 40 is wound counterclockwise,while the second coil 42 is wound clockwise. The lowermost turn of eachcoil is connected to electrical ground. The uppermost turn of each coilis connected to the output 35 of an unbalanced impedance matchingnetwork 31, which receives its input from RF power supply 32. Becausethe two coils 40, 42 are wound in opposite directions, they producerespective magnetic fields of opposite polarity when driven by the sameRF current.

In the embodiments of FIGS. 9 and 10, both coils 40 a and 42 are woundin the same direction, which is illustrated as clockwise but could becounterclockwise just as suitably. The two coils are connected in seriesby connecting the lowermost turn of the first coil to the lowermost turnof the second coil at junction 39.

In the FIG. 9 design, the RF power supply 32 is connected to the twocoils of each pair through an impedance matching network 31 a whichprovides a balanced (i.e., differential) output signal between the twooutputs 48, 49. The first output terminal 48 of the matching network isconnected to the uppermost turn of the first coil 40 a, and the secondoutput terminal 49 is connected to the uppermost turn of the second coil42. The two coils 40 a and 42 produce respective magnetic fields ofopposite polarity because the direction of current flow in one coil isopposite the direction of current flow in the other coil. The lowermostturn of each coil 40 a, 42 (connected together at junction 39) is atelectrical ground potential because the two outputs of the matchingnetwork are balanced relative to the grounded transformer center tap.

A possible modification (not shown) of the FIG. 9 design would be toeliminate the center tap of the transformer secondary winding, in whichcase the induction coils 40 a, 42 would be “floating” relative toground, i.e., no path would exist to connect the secondary winding andthe induction coils to ground. In this alternative “floating”implementation, the lowermost turn of each coil and junction 39 stillwould be maintained close to electrical ground potential because thelayout of the coil array is symmetrical relative to the balanced outputs48, 49 of the impedance matching network.

In the FIG. 9 design, the impedance matching network 31 a employs atransformer 38 to convert the unbalanced output of the power supply 32to a balanced output 48, 49. A disadvantage of the FIG. 9 design is thatit is difficult to design an efficient, high power, RF transformer,i.e., one which can withstand high RF voltages without arcing, which hassufficiently low resistance to achieve high efficiency, and which has ahigh coefficient of coupling between the primary and secondary windings.

FIG. 10 shows a novel circuit for driving each induction coil pair 110so that the junction 39 between the two coils 40 a, 42 of each pair isapproximately at electrical ground potential. In contrast with the FIG.9 design which requires an impedance matching network 31 a having a pairof balanced outputs 48-49, the FIG. 10 design works with anyconventional impedance matching network 31 having an unbalanced output35.

Specifically, in the FIG. 10 design, the “hot” end of the firstinduction coil 40 a (i.e., the end of the first coil 40 a which is notconnected to the second coil 42) is connected to the output 35 of thematching network 31, and the “hot” end of the second induction coil 42is connected to electrical ground through a capacitor 90. The capacitor90 has a capacitance value chosen to create a series resonance with theinductance of the second induction coil 42 at approximately thefrequency of the RF power supply 32. Because of this series resonance,the junction 39 between the two induction coils is close to electricalground. Since the capacitive reactance of capacitor 90 approximatelycancels out the inductive reactance of the second coil 42, the impedanceof the load connected to the output 35 of the impedance matching network31 is approximately the inductance of the first coil 40 a plus the loadimpedance of the plasma (plus a smaller resistive component due to heatdissipated in the ferrite cores).

Because the induction coil pairs 110 are connected in parallel with eachother, the inductance with which the capacitor 90 should resonate at thefrequency of the RF power supply is the parallel combination of therespective second coils 42 of every coil pair 110. If there are Nidentical coil pairs, the combined inductance of the N second coils 42will be the inductance of one second coil 42 divided by N. Consequently,the required capacitance value for capacitor 90 will be N multiplied bythe capacitance which would resonate with one of the second coils 42 atthe frequency of the RF power supply. (In the preferred embodiment ofFIGS. 1-7, there are four coil pairs 110, hence N=4.)

While any conventional impedance matching network 31 can be used, FIGS.8 and 10 show our preferred circuit for the matching network, which is aconventional “L network”. The network transforms the plasma loadimpedance to match the 50 ohm resistive output impedance of the powersupply 32. The variable “load” capacitor 91 and the variable “tuning”inductor 93 preferably are adjusted by a feedback control loop tominimize the reflected power at the output of the RF power supply 32, asmeasured by a conventional reflected power detector. When the network isadjusted for minimum reflected power, the inductance of the “tuning”inductor 93 generally will be adjusted so that the series combination ofthe three capacitors 90, 91, 92 and the three inductors 40 a, 42, 93resonates at approximately the frequency of the RF power supply 32. Theinput impedance into which the plasma load impedance is transformed isdetermined primarily by the adjusted value of the “load” capacitor 91.

Alternatively, the fixed capacitor 92 can be replaced with a variable“tuning” capacitor, in which case the “tuning” inductor 93 can beeliminated. However, we consider a variable inductor preferable to avariable capacitor for this high voltage application, because a suitablevariable capacitor would be much more expensive. Also, the tuning shaftof a typical high voltage variable capacitor has high rotationalfriction which impedes rapid tuning of the impedance matching network.

In contrast, the “load” capacitor 91 has a much lower impedance, henceneed not withstand such high voltages and can be procured or fabricatedeconomically. Our preferred design for the variable “load” capacitor isdescribed in commonly assigned application Ser. No. 08/954,376 filedOct. 20, 1997 by Richard Mett et al. entitled “High Efficiency ImpedanceMatching Network,” the entire contents of which are hereby incorporatedinto this patent specification.

Our presently preferred embodiment of the coil array 30 employs eightinduction coils (i.e., four coil pairs), where each of the eight coilshas an inductance of 3.6 μH. In the designs of FIGS. 9 and 10 in whichpairs of induction coils 40 a, 42 are connected in series and the fourpairs are connected in parallel, the total inductance of the coil arrayconnected between the capacitor 90 and the output 35 of the impedancematching network is 1.8 μH.

Our presently preferred embodiment uses the circuit of FIG. 10 in whichthe frequency of the RF power supply is 13.56 MHz; the capacitor 90 is120 pf; the load capacitor 91 is a 450 pf fixed capacitor in parallelwith a 30-1300 pf variable capacitor; the tuning capacitor 92 is 90 pf;and the tuning inductor is 0.35-0.70 μH.

As stated earlier, each induction coil 40 a, 42 in the preferredembodiment has 3¾ turns of wire. In an earlier prototype, each inductioncoil had 6 turns of wire. We found that stray capacitance of the coilarray in the earlier prototype gave the array a self-resonant frequencyof about 14 MHz, almost identical to the 13.56 MHz frequency of the RFpower supply 32. Consequently, it was very difficult for any impedancematching network to match the coil array to the power supply outputimpedance. Reducing the number of turns to the present 3¾ turns raisedthe self-resonant frequency to about 28 MHz. This experience illustratesthat the optimum design will be affected by parasitic inductance andcapacitance in the physical layout of the coil array.

Each pair of series-connected coils, (40, 42) or (40 a, 42), optionallymay topped by a magnetic shunt (not shown) which magnetically connectsthe magnetic cores of the two coils of the pair. Such a shunt would beexpected to increase somewhat the magnetic flux produced by the coils.However, in our experiments the shunt did not produce any noticeablebenefit, so no shunt is included in the preferred embodiment.

As stated earlier, the induction coil array shown in FIGS. 1-3 producesexcellent spatial uniformity of the tested semiconductor fabricationprocess in spite of the gap in the center of the array in which there isno induction coil. Nevertheless, there may be applications in which thespatial uniformity of the process can be improved by adding a coil inthe center of the array to alter the magnetic field pattern. FIGS. 11and 12 show an alternative induction coil array which adds a cylindricalcenter coil 60 wound around a Teflon dielectric coil form 62 in theshape of a cylinder whose axis is parallel to the axes of thesurrounding coils 40, 42. A cylindrical ferrite magnetic core 64preferably occupies a cylindrical cavity in the center of the coil form62.

The center coil preferably is connected to the same RF power supply 32which supplies power to the wedge-shaped coils 40, 42. It does notmatter whether the center coil 60 is connected in phase with the S coils40 or in phase with the N coils 42. The magnetic field strength of thecenter coil can be adjusted to produce a desired magnetic field pattern.Preferably, the field strength of the center coil is empiricallyadjusted to maximize the spatial uniformity of the magnetic field or tomaximize the spatial uniformity of the fabrication process performed onthe workpiece. The magnetic field strength can be adjusted by changingthe number of turns of wire in the center coil 60, by changing theamount or composition of the ferrous material used in the core aroundwhich the coil is wound, or by adjusting the amount of RF power appliedto the center coil relative to the other coils.

FIG. 13 shows yet another alternative embodiment of an induction coilarray having two concentric arrays of coils. Specifically, in thisembodiment, each of the wedge-shaped coils of the FIG. 2 embodiment isdivided into a radially inner coil 70 and a radially outer coil 72, witha gap between them. The coils are connected to the first RF power supplywith polarities selected so that adjacent coils in any direction willhave opposite polarities, as indicated by the S and N symbols in FIG.13.

The concentric coil embodiment of FIG. 13 affords a greater number ofphysical parameters that can be adjusted independently to maximize thespatial uniformity of the process being performed on the semiconductorworkpiece, or to optimize some other process performance parameter. Forexample, dimensions which can be adjusted independently include theazimuthal gap between adjacent inner coils 70, the azimuthal gap betweenadjacent outer coils 72, and the radial gap between an inner coil 70 andits adjacent outer coil 72. Because the interactions among theseparameters are complex, the adjustments typically would be optimizedempirically.

The concentric coil embodiment of FIG. 13 produces a magnetic fieldhaving a cusp pattern whose radial component is comparable in magnitudeto its azimuthal component. In contrast, the FIG. 2 embodiment producesa magnetic field having a negligible radial component. This distinctionaffects the uniformity of the plasma produced in the process chamber,because a radially oriented magnetic field causes azimuthal drift ofelectrons in the plasma, thereby contributing to azimuthalnon-uniformity of plasma density. Conversely, an azimuthally orientedmagnetic field causes radial drift of electrons in the plasma, therebycontributing to radial non-uniformity of plasma density. The choicebetween the embodiments of FIGS. 2 and 13 may depend on the need tooffset other chamber design and chemical process factors that influencespatial uniformity.

The preferred embodiments discussed above employ an RF power supply 32which powers the coil array at a frequency of 13.56 MHz. Our theoreticalanalysis predicts that lowering the RF frequency would increase thecurrent in the coils for a given power level, which would increase thepower dissipated as heat due to the resistance of the wires of theinduction coils. The increased current also would increase the inducedmagnetic field, which would increase the power dissipated as heat in theferrite cores. We tested this theory by changing the power supplyfrequency from 13 MHz to 2 MHz. Our tests confirmed that the ferritecores were hotter at 2 MHz than at 13 MHz for the same RF power levelbeing delivered by the power supply.

Our analysis also predicts that lowering the RF frequency would increasethe penetration distance of the magnetic field. For a given workpiecemounting position relative to the coil array, increasing the penetrationdistance generally will increase the process rate and decrease thespatial uniformity of the process, as described earlier.

In the described preferred embodiments, the process gases are excited toa plasma state by a combination of the RF power inductively coupled fromthe coil array 30 and the RF power capacitively coupled from the cathodeelectrode 16. Our inventive coil array also can be used to enhance thedensity of a plasma excited by a plasma source of any other type.

What is claimed is:
 1. A method of inductively coupling electrical powerto the interior of a plasma chamber, comprising the steps of: mounting aplurality of wedge-shaped induction coils adjacent an interior region ofthe plasma chamber, the coils being mounted so as to position each coilrelative to a geometric surface having a circular transverse section sothat an axial end of each coil is positioned adjacent the geometricsurface, the coils are equally spaced azimuthally relative to thegeometric surface, and the adjacent sides of any two adjacent coils areapproximately parallel to a radius of the circular transverse section ofthe geometric surface; and supplying electoral alternating current tothe coils so that the coils produce a magnetic field.
 2. A methodaccording to claim 1, wherein the step of supplying current furthercomprises: supplying said electrical current to the respective inductioncoils in respective polarities such that any two adjacent inductioncoils produce respective magnetic fields of opposite polarity.
 3. Amethod according to claim 2, further comprising the step of: for any twoadjacent coils, winding one of said two adjacent coils clockwise, andwinding the other one of said two adjacent coils counterclockwise.
 4. Amethod according to claim 2, wherein the step of supplying currentfurther comprises: winding each of the coils in the same direction;supplying said alternating current between two power supply outputs; andconnecting any two adjacent coils to the two power supply outputs inopposite polarities so that the supplied electrical current flowsthrough said two adjacent coils in opposite directions.
 5. A methodaccording to claim 1, wherein the geometric surface of a surface of acircular dielectric wall of the chamber.
 6. A method according to claim1, further comprising the step of: holding a semiconductor workpiece ata position in the chamber which is a distance from the induction coils;wherein the step of mounting the induction coils further comprisesspacing apart azimuthally adjacent induction coils by an azimuthal gapwhich is sufficiently small, relative to the distance between the coilsand the workpiece position, so that the strength of the magnetic fieldproduced by the coils is at least ten times smaller at the workpieceposition than at another position within the plasma chamber which iscloser to the coils.
 7. A method of inductively coupling electricalpower to the interior of a plasma chamber, comprising the steps of:holding a semiconductor workpiece at a generally planar, circularworkpiece area having a central axis, wherein the workpiece area isinside the plasma chamber; mounting a plurality of wedge-shapedinduction coils adjacent an interior region of the plasma chamber,wherein each respective coil is wound around a respective axis which isgenerally parallel to said central axis, the coils are equally spacedazimuthally around said central axis, and each coil has a transversesection which is wedge-shaped so that, for every two adjacent coils, therespective adjacent sides of said two adjacent coils are approximatelyparallel to a radius of the circular workpiece area; and supplyingelectrical alternating current to the coils so that the coils produce amagnetic field.
 8. A method according to claim 7, wherein the step ofmounting the induction coils further comprises the steps of: orientingeach induction coil so that an axial end of the coil faces the workpiecearea; and mounting the induction coils so that the respective axial endof all of the induction coils are positioned in a plane which isparallel to the workpiece area.
 9. A method according to claim 7,wherein the step of supplying current further comprises: supplying saidelectrical current to the respective induction coils in respectivepolarities such that any two adjacent induction coils produce respectivemagnetic fields of opposite polarity.
 10. A method according to claim 7,further comprising the step of: for any two adjacent coils, winding oneof said two adjacent coils clockwise, and winding the other one of saidtwo adjacent coils counterclockwise.
 11. A method according to claim 7,wherein the step of supplying current further comprises: winding each ofthe coils in the same direction; supplying said alternating currentbetween two power supply outputs; and connecting any two adjacent coilsto the power supply outputs in opposite polarities so that the suppliedelectrical current flows through said two adjacent coils in oppositedirections.
 12. A method according to claim 7, wherein the step ofmounting the induction coils further comprises: spacing apartazimuthally adjacent induction coils by an azimuthal gap which issufficiently small, relative to the distance between the coils and theworkpiece, so that the strength of the magnetic field produced by thecoils is at least ten times smaller at the workpiece than at anotherposition within the plasma chamber which is closer to the coils.
 13. Amethod according to claim 7, further comprising the steps of: providingan additional induction coil having a winding axis; mounting theadditional induction coil so that its winding axis is coincident withthe central axis of the workpiece area; and supplying electricalalternating current to the additional coil so as to produce a magneticfield.
 14. A method of inductively coupling electrical power to theinterior of a plasma chamber, comprising the steps of: providing aplurality of induction coils, wherein each coil has a longitudinal axisaround which that coil is wound; positioning the induction coils in arectangular matrix adjacent an interior region of the plasma chamber,the coils being positioned so that each coil has an axial end positionedadjacent a geometric plane, the longitudinal axis of each coil ispositioned perpendicular to the geometric plane, and the coils arespaced apart from each other so that the transverse gap between therespective perimeters of any two adjacent coils equals a fixedpredetermined distance which is substantially identical for any twoadjacent coils; and connecting an RF power supply to supply electricalpower to the induction coils so that the coils produce a magnetic fieldin a portion of said interior, the RF power supply being connected tothe respective induction coils in respective polarities such that anytwo adjacent induction coils produce respective magnetic fields ofopposite polarity; wherein each of the radially outermost ones of saidcoils is connected to the RF power supply so that every two adjacentones of said radially outermost coils produce respective magnetic fieldsof opposite polarity; and wherein each of the radially outermost ones ofsaid coils is positioned so that every two adjacent ones of saidradially outermost coils is separated by said predetermined distance.15. A method according to claim 14, wherein the providing step furthercomprises providing said coils having a circular transverse crosssection.
 16. A method according to claim 14, wherein: the providing stepfurther comprises providing said coils having a rectangular transversecross section; and the positioning step further comprises positioningsaid coils so that adjacent sides of adjacent coils are parallel.
 17. Amethod according to claim 14, further comprising the step of: holding asemiconductor workpiece parallel to the geometric plane.
 18. A methodaccording to claim 14, further comprising the step of: holding asemiconductor workpiece at a position in the chamber which is a distancefrom the induction coils; wherein the fixed predetermined distance issufficiently small, relative to the distance between the coils and theworkpiece position, so that the strength of the magnetic field producedby the coils is at least ten times smaller at the workpiece positionthan at another position within the plasma chamber which is closer tothe coils.
 19. A method according to claim 18, further comprising thestep of: holding a semiconductor workpiece parallel to the geometricplane.
 20. A method according to claim 18, further comprising the stepof: mounting within each induction coil a magnetic core having amagnetic permeability substantially greater than one.
 21. A methodaccording to claim 1, wherein the step of mounting the plurality ofwedge-shaped induction coils further comprises: mounting the coils sothat the adjacent sides of any two adjacent coils are parallel to eachother.
 22. A method according to claim 7, wherein the step of mountingthe plurality of wedge-shaped induction coils further comprises:mounting the coils so that the adjacent sides of any two adjacent coilsare parallel to each other.